1. Field of the Invention
The present invention relates to flash point detection and, more particularly, to a method and apparatus for optical flash point detection in an open cup tester.
2. Description of the Prior Art
The term “flash point” is the lowest temperature at which a volatile substance can be vaporized into a flammable gas. To measure the flash point, an ignition source has to be introduced to the substance with the “flash” being the point at which the vapor is ignited. While there are various methods of measuring the flash point, those methods can generally be divided into two main categories: open and closed cup flash points.
Measuring a flash point using an open cup method is conducted in a vessel which is exposed to the air outside. Temperature of a substance being tested is rapidly raised until the temperature approaches the theoretical flash point, after which the temperature is gradually raised with an ignition source being periodically passed over the top of the substance. When the substance reaches a temperature at which it “flashes” and ignites, the true “flash point” has been reached. The most commonly-used open cup method is called the Cleveland open cup. There is a published standard by the American Society of Testing and Materials (ASTM) entitled “Standard Test Method for Flash and Fire Points by Cleveland Open Cup Tester,” ASTM D92-16A. The flash point of a substance may vary according to the distance between the substance being measured and the ignition source, i.e., the height of the flame above the cup.
The closed cup flash point detector, as the name suggests, uses a closed cup with the substance being tested being inside a closed container which is not open to outside atmosphere. A lid is sealed in place over the substance being tested and the ignition source is introduced into the vessel itself. There are four general kinds of closed cup flash point detectors, Pensky Martens, Abel, Tag and Small Scale (also known as Setaflash).
As automated test equipment started being developed, the industry adopted an ionization ring as a method of detecting the flash point. The ionization ring would sit inside the test cup between the sample and the ignition source. The ionization ring would sense a charge caused by the ignition. There were drawbacks to using the ionization ring method of detection of flashpoint. Some samples being measured expand as they are heated, which expansion may cause contact with the ionization ring and give a false signal. Samples with high water content also give a false flash point signal. Other samples such as silicon oils produce vapors that coat the ionization ring, which insulates the ionization ring so that it does not sense the ignition. Failing to sense the ignition creates a safety issue because the detector will continue to heat the sample.
In using the Cleveland open cup method, a test cup (usually brass) is filled to a certain level with the substance being measured. Then, the temperature of the substance being measured is increased rapidly until it approaches the theoretical flash point. Thereafter, the temperature is increased at a slow constant rate as it approaches the theoretical flash point. The increase in temperature of the substance being tested will cause the substance to produce flammable vapor in increasing quantities and density. The lowest temperature at which a small test flame passed over the surface of a liquid causes the vapor from the liquid to ignite is considered the substance flash point.
The “fire point” of a substance is the lowest temperature at which the vapor of that substance will continue to burn for at least five (5) seconds after ignition by an open flame. At the “flash point,” a lower temperature, a substance will ignite briefly, but vapor is not being produced at a rate to sustain the fire. Most tables of material properties only list the flash points. Generally, the fire points of a substance are about ten degrees Centigrade (10° C.) higher than the flash points.
A related patent over which this invention is an improvement is U.S. Pat. No. 5,932,796 issued on Aug. 3, 1999, entitled “Apparatus for the Determination of a Flash Point of a Substance” by Arthaud et al., which related patent is incorporated by reference. However, the '796 patent has some problems that affect its accuracy. The flame 5 is located some distance above the substance 2 being tested, which affects the measurement. Further, the flame 5 may cause false readings indicating that a flash point has been reached.